696 Wisconsin Academy of Sciences, Arts, and Letters. 
of starch of uniform thickness is crystallized; if, on the other 
hand, the plastid is thicker on one part of the grain, more 
material will be added to the peripheral layer at that part, 
and if we consider the mother substance of this layer to be too 
viscid to allow the added material to spread readily to the op¬ 
posite end of the grain, the thicker parts of the layer will be 
deposited at the part of the grain where the plastid is thickest. 
All available evidence seems to favor this hypothesis as an ex¬ 
planation of the peripheral layer and the method of growth 
of the grain. 
Although in many cases it is impossible to discover the 
plastid on the starch grain in thin sections, it is probable that 
it remains on the grain through the stages of growth and solu¬ 
tion. It is exceedingly thin and oftentimes is removed in the 
preparation of the slide. 
Salter claims to have found plastids on all the grains of 
Pellionia , but on the material from the potato he was unable 
to find the plastid in every case. 
By a careful series of observations on the same grain 
treated successively with different reagents, I have convinced 
myself that in addition to more or less dense layers of starch, 
there are sharp lines, as shown in Figure 43, 1 and 2, which 
mark the boundaries of highly refractive layers, and also 
spaces or open water-filled crevices which widen or become 
more narrow with the swelling and contraction of the adjacent 
dense layers. 
The layers which take the deepest color with gentian violet 
and iodine are in general the densest layers. They are highly 
refractive when mounted in water, and stains are removed 
from them with difficulty. In the case of precipitates formed 
in the grain, of course the conditions are just the opposite of 
those in ordinary staining, and, as described above, there is no 
question that the zones of granules, formed by precipitating 
methyl violet with calcium nitrate or with picric acid, lie in 
the more open watery strata and in the crevices between the 
denser strata of the grain. 
Meyer holds that the loose and not the dense layers take the 
deepest stains, and this view has since been accepted by Sal- 
